GB2084818A

GB2084818A - Apparatus for controlling magnetic field

Info

Publication number: GB2084818A
Application number: GB8128470A
Authority: GB
Original assignee: Jeol Ltd; Nihon Denshi KK
Current assignee: Jeol Ltd
Priority date: 1980-09-26
Filing date: 1981-09-21
Publication date: 1982-04-15
Also published as: GB2084818B; JPS6248345B2; US4417145A; JPS5760647A

Description

1 GB 2 084 818A 1

SPECIFICATION

An apparatus for controlling magnetic field intensity

This invention relates to an apparatus for controlling the magnatic field intensity of electromag- 5 netic lenses or charged particle beam deflection means used in a charged particle beam apparatus such as an electron microscope, an X-ray micro-analyser, a device for electron beam exposure, or other similar charged particle beam devices.

Electromagnetic lenses and deflecting means in an X-ray micro-analyser and the like incorporate ferromagnetic yokes disposed adjacent to the excitation coils through which the 10 excitation current flows. Different lens powers (focal lengths) or deflection powers are obtained under some circumstances for a given excitation current supplied to given lens coils or deflection coils, because of the hysteresis of the ferromagnetic yokes. Therefore, it is necessary to remove the effect of the hysteresis of the lens or deflection means, particularly when the apparatus is to be controlled by a computer.

An object of the present invention is to provide a novel apparatus for controlling an electromagnetic lens or deflecting means so as to reduce effects due to the hysteresis of the ferromagnetic yoke.

According to the invention there is provided an apparatus for controlling the magnetic field intensity of a ferromagnetic yoke adjacent to an excitation coil which is energised by an 20 excitation current source, the apparatus comprising; a) a field intensity specifying means for generating a specifying signal corresponding to the desired magnetic field intensity, and b) a control signal generating means having a memory means which stores the hysteresis data of the ferromagnetic yoke for generating control signals to be applied to the said excitation 25 current source; the said control signals being determined by the specifying signal supplied anew from the said means a) and the previous specifying signals supplied from the said means a).

The invention will further be described with reference to the accompanying drawings, of which:- Figure 1 is a block digram of control apparatus according to this invention, and Figures 2 and 3 are graphs for explaining the principle of the invention.

Fig. 1 shows one embodiment of the invention. In the figure, a lens coil 'I a is surrounded by ferromagnetic yoke 1 b. The lens coil 1 a is supplied with the output from an excitation current source 2 for the lens; which excitation current source 2 is controlled by a control signal from a control signal generating means 4 to which the output from a field intensity specifying means 3 35 for specifying a lens power (focal length or magnetic field intensity) is supplied. The means 3 produces an output signal B corresponding to a magnetic flux density B in gaps between magnetic pole pieces of the ferromagnetic yoke 1 b for specifying a desired focal length of the lens in accordance with the excitation current source 2. The control signal generating means 4 comprises a reversed signal-vaiue holding means 5 for detecting and storing an updated signal 40 value BO when the value of specifying signals succesively applied from the specifying circuit 3 is changed from an increasing to a decreasing trend or from a decreasing to an increasing trend; a target-specifying-signai-value holding means 6 for holding an updated specifying signal B2; and a current-specifying-signal-value holding means 7 for holding a specifying signal value B1 immediately before the updated specifying signal is applied. A central processing unit 8 collates 45 the signals BO, B1 and B2 with data stored in a memory means 9 and, based on the results, supplies a control signal to the excitation current source 2 to supply the lens coil 1 a with such an excitation current as to generate a magnetic flux corresponding to the specifying signal B2.

The principles of operation of the apparatus shown in Fig. 1 will be described. Fig. 2 shows a hysteresis characteristic curve called a minor loop for the ferromagnetic yoke 1 b with the 50 hoizontal axis indicating the intensity W of the excitation current supplied to the lens coil 1 a and the vertical axis indicating the magnetic flux density J3 in the magnetic lens or the output B from the field intensity specifying means 3 which corresponds to the magnetic flux density (in tensity)

B. It is now assumed that the specifying means 3 is actuated to change the signal so that it is 55 increased from a value below bO up the value bO at a point of reversal, is next reduced down to the current value bl (i.e. the excitation current]CL), and then reaches the target value b2, as indicated by the curve PM At this time, the means 5, 6 and 7 of Fig. 1 hold specifying values bO, b2 and bl, respectively, (condition 1 in Table 1) as described above.

GB2084818A 2 Table 1

Conditions 1 11 Ill N Reversal-value holding circuit 5 bO Target-specifying-value holding circuit 6 b2 Current-specifying-value holding circuit 7 bl bO 1J3 1J2 1J3 b2 bl b2 b3 b2 The memory means stores previously measured data the content of which is shown in Fig. 3. In Fig. 3, the horizontal axis indicates a change (AB) in the specifying signal, including polarity, with the point of zero being the value of reversal when the output from the specifying means 3 is changed from an increasing to a decreasing trend or from a decreasing to an increasing trend.

The vertical axis indicates at A1 a change in the excitation current for the lens coil necessary to give AB. The characteristic curve illustrated in Fig. 3 is intrinsic in electromagnetic lenses, and is measured by using the tens actually used. Since the specifying signal is caused to change from the value bl to the value 1J2 with the reversal value bO at the origin of AB under the said condition 1, the central processing unit 8 computes such that AI should only be changed for an amount of A] 1 shown in Fig. 3. As a result, the control signal of All is supplied to the excitation 20 current source 2, causing the excitation current to vary from IQ to IR.

When the output from the specifying means 3 is continuously reduced to the specifying value H (condition 11) after the condition 1, the excitation current is changed by A12 to IS.

Under the condition Ill in which the output of the specifying means 3 is returned again to the specifying value b2, the specifying value is changed from JJ2 to H to 1J2 and hence the value 25 of the signal stored in the reversal signal value holding means 5 is changed from bO to b3, whereupon the value of the signal held in the target signal value holding means 6 becomes b2, and the value of the signal held in the current specifying signal value holding means 7 becomes H. Therefore, the central processing unit 8 computes a change A13 in the excitation current corresponding to a change (b2-b3) from the origin of H in Fig. 3 and, based on the result, 30 supplies a control signal to the excitation current source 2, causing the excitation current to take the value of IT in Fig. 2.

Meanwhile, under the condition N in which the output from the specifying means is changed from bO to b2 and then to the new specifying value bl, the reversal- signal-value holding means 5, the target-specifying-signal-value means 6 and the current-specifyingsignal-value holding means 7 hold the signals b2, bl and 1J2 respectively. Therefore, the central processing unit 8 computes a change A14 in the excitation current corresponding to a change (b 1 -b2) from the origin of 1J2 in Fig. 3 and, based on the result, supplies a control signal to the excitation current to take the value of [V in Fig. 2.

As described above, weakened excitation is carried out by controlling the excitation current 40 along an upwardly convex portion (for example, PGR) of the hysteresis curve of Fig. 2, and increased excitation is carried out by controlling the excitation current along downwardly concave portions (for example, RV, STU) of the hysteresis curve, whereby a desired lens power can be reached with precision without being adversely affected by hysteresis of the ferromag- netic yoke.

When the apparatus shown in Fig. 1 is started up, it is necessary to effect a special operation manually or automatically to maintain a starting point of the hysteresis curve shown in Fig. 2 at a constant point.

The present invention is applicable to an electro-magnetic deflecting means which serves as a means for deflecting a beam of electrons in an X-ray micro-analyser or the like, as well as to an 50 electromagnetic lens. More specifically, on condition that AI is a current supplied to the electromagnetic deflecting means and AB is a magnetic flux density corresponding to an amount by which an electron beam is to be deflected, the electron beam can be shifted from one point to another with precision without being adversely affected by hysteresis of a magnetic yoke of the electromagnetic deflecting means and a yoke of an electromagnetic lens around the 55 electromagnetic deflecting means.

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Claims

1. An apparatus for controlling the magnetic field intensity of a ferromagnetic yoke adjacent to an excitation coil which is energised by an excitation current source, the apparatus comprising:

a) a field intensity specifying means for generating a sepcifying signal corresponding to the desired magnetic field intensity, and b) a control signal generating means which stores the hysteresis data of the ferromagnetic yoke for generating control signals to be applied to the said excitation current source; the said 65 Z 3 GB2084818A 3 control signals being determined by the specifying signal supplied anew from the said means a) and the previous specifying signals supplied from the said means a).

2. An apparatus as defined in claim 1, wherein the said control signal generating means comprises:

c) a target-specifying-value holding circuit for holding the newest output signal from the said 5 specifying means a), d) a current-specifying-value holding circuit for holding the current specifying signal occur ring immediately before the said target specifying signal is produced by the said specifying means, and e) a reversal-value holding circuit for holding the specifying signal as it changes from an 10 increasing to a decreasing trend or from a decreasing to an increasing trend before the said target specifying signal is produced.

3. An apparatus for controlling magnetic field intensity substantially as hereinbefore de scribed with reference to the accompanying drawings.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd-1 982. Published at The Patent Office, 25 Southampton Buildings, London, WC2A l AY, from which copies may be obtained.